Off gases from many organic and inorganic processes comprise a mixture of HCl, HF, uncondensed organics and inorganics, which must be neutralized or otherwise disposed of in an environmentally and socially acceptable manner. Presently, the preferred method for disposing of these gases is subjecting them to a neutralization process. However, this is not an acceptable long term solution since neutralization processes produce soluble and insoluble chlorides and fluorides which may contribute to water pollution problems, or produce a sludge which must subsequently be disposed of in a landfill.
In examining alternatives to neutralization processes; social, ecological, and conservation considerations must be taken into account along with economic considerations. In this respect, one of the least desirable aspects of neutralization processes is that the neutralization products of HCl and IF are discarded. Although the combined HF/HCl off gas has little apparent industrial use, and is therefore of little commercial value, HCl and HF are individually useful for many industrial processes. Therefore, an optimum alternative system for the recovery of HF/HCl off gases would separate the HF from the HCl so that each component would be reusable in some commercial, industrial process. For this to be feasible, however, the cost of the recovery system must be such that the cost of regenerating HCl and HF is at least marginally competitive with HCl and HF available from present commercial sources. Furthermore, the HF and HCl must be separated from whatever other impurities are present in the off gases.
Aside from delivering economically affordable HF and HCl, the recovery system should be fairly simple to operate; avoid the use of supplementary extractants which complicate the process and add to operating costs; and operate at atmospheric or moderate pressures. In addition, the system should use standard, low cost construction materials. For example, dry HCl and anhydrous liquid or gaseous HF is currently handled in steel equipment. Introduction of any moisture obviously can cause severe corrosion problems, resulting in the need for high-cost corrosion resistance materials. The recovery system should be designed to avoid such problems.
Although many HF recovery processes have been proposed as substitutes for neutralization processes, none of them have the criteria for successful industrial applications. For example, in one proposed method vent gases containing HF which originate from a fluorination process to produce refrigerants are recovered by scrubbing the gases with concentrated sulfuric acid. This process is based on the absorption-desorption equilibrium, H.sub.2 SO.sub.4 +HF HSO.sub.3 F+H.sub.2 O. Pursuant to this method, vent gas containing HF, HCl and uncondensed organics is introduced at the bottom of a tower. Ninety eight percent concentrated sulfuric acid is sprayed at the top of the tower at ordinary temperature. The gaseous effluent leaving the top of the tower is introduced into an entrainment separator, and then washed with water to dissolve HCl. The gas is then cooled to 6.degree. C. under a pressure and the organic product is separated by liquefaction. The concentrated sulfuric acid leaving at the bottom of the tower is heated to 120.degree. C. and is recycled to the top of the tower after having been separated from HF, thus regenerating the absorbent. The major problem with this recovery scheme is that the HF--HCl--H.sub.2 SO.sub.4 --H.sub.2 O environment is a highly corrosive atmosphere and consequently requires that the equipment would be made of expensive corrosion resistant materials. In addition, the recovered HF is contaminated with water and sulfuric acid originating from the stripping operation, and the HCl would be impure.
Another suggested process involves absorption of HF directly in fluosulfonic acid (HSO.sub.3 F). Many variations of this principle have been proposed, but all face the same corrosion and purity problems mentioned above.
In a third method, thirty eight percent muriatic acid is used to scrub out HF. A drawback to this technique is that the HF is recovered in an aqueous solution, from which it is difficult to isolate anhydrous HF. Although other absorbents are also suggested, such as fluorinated solvents, diphenylamine, benzophenone and dihydroxyfluoroboric acid, none of these can produce the HCl purity required because these absorbents act as contaminants in the HF and HCl.
The complexity of this extraction process is better appreciated by an example in which extraction is proposed from an aqueous solution with a secondary amine in a hydrocarbon solvent which is in turn extracted with aqueous KF in three stages to form KHF.sub.2, and is then crystallized and calcined at 500.degree. C. to produce anhydrous HF.
In another proposed method a gaseous mixture of HF and HCl is scrubbed in a jacketed tower with an aqueous solution containing H.sub.3 BO.sub.3 and sufficient HCl to give partial pressure of HCl in the solution equal to that in gas mixture. The HF reacts with the H.sub.3 BO.sub.3 to form HBF.sub.4 which is retained in the scrubber liquor. Here previous problems of working with aqueous media are repeated in addition to the introduction of H.sub.3 BO.sub.3 which could itself become a contaminant in the recovered HF.
Some of the proposed recovery processes are quite complex. For example there is one in which ammonium fluoride compounds are formed with HF. From the ammonium fluoride salt HF is recovered by thermal decomposition to obtain a gas containing ammonia and hydrogen fluoride. This is followed by further thermal decomposition of the ammonia to hydrogen and nitrogen and subsequently recovering hydrogen fluoride from the gaseous mixture. A modified process of recovering anhydrous hydrogen fluoride from aqueous ammonia fluoride solutions comprises heating the ammonium fluoride solution to a temperature sufficient to volatilize substantially all of the water and part of the ammonia to thereby form ammonium bifluoride. This ammonium bifluoride is then reacted with an alkali metal fluoride to produce an alkali metal bifluoride and HF is recovered by heating this alkali metal bifluoride to a temperature sufficient to yield an alkali metal fluoride and HF. No mention is made of HCl, which would yield ammonium chloride, and thus complicate the process further.
Other methods proposed include reaction with Al.sub.2 O.sub.3, Al(OH).sub.3, or other reactants. In the former case H.sub.2 O is a by-product with the net result that the problems encountered with aqueous media would be inherited. In this process, AlF.sub.3 is formed and the reconversion to HF would have to be done chemically, which makes it economically unattractive. Other recovery processes envisioned are ion exchange, reaction with silica gel, and use of alkali fluorides. In this case the alkali bifluorides formed are then thermally decomposed. The workability of such systems is questionable and with the latter, high HF losses with HCl would result.
A procedure which avoids use of added reagents is high pressure fractionation. In a typical embodiment of such a system the HF-HCl off-gas is compressed to 225 psig in a 2 stage compressor system. The HCl-HF mixture is liquefied at a temperature of -20.degree. C. The resultant condensate is fed to a still operated at 225 psig to separate HF and HCl. There are many problems to operating such a process. Some of the difficulties in operation of a compression system are choice of resistant lubricants with HF and HCl, and decomposition of contained organics and selection of seals, gaskets and valve packing. Also all equipment, including storage tanks, would have to have a high pressure rating.
Still another HF recovery system suggests that a mixture of gaseous HF, HCl and chloro-fluorocarbons (paraffinic) be separated by anhydrous fractional distillation at -50.degree. to -40.degree. C. and 100 psig. to distill off HCl, settling the remaining mixture until it forms a layer of substantially anhydrous HF containing a minor portion of dissolved organic products and substantially pure organic layer. This process is susceptible to the many problems of a pressure fractionation process as noted.